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Abstract:

Provided is an optical scanning apparatus has a plurality of light
emitting portions and an incident optical system including optical
element, wherein each shape of optical surfaces in a main scanning
section of optical elements is formed into a noncircular shape. When
defining that W is a space between specific light emitting portions
farthest from an optical axis in the main scanning direction, La is an
optical path length between an aperture stop and a specific optical
surface closest to the light source unit among the noncircular optical
surfaces of the incident optical system, f1 is a focal length of the
incident optical system in the main scanning direction, and D is a light
flux width in the main scanning direction of a light flux emitted from
the specific light emitting portion in the main scanning direction on
specific the optical surface, the equation
2D≧|WLa/2f1|≧D/8 is satisfied.

Claims:

1. An optical scanning apparatus, comprising:a light source unit including
a plurality of light emitting portions that are spaced apart from one
another in a main scanning direction;a deflection unit for deflecting a
plurality of light fluxes emitted from the plurality of light emitting
portions for scanning;an incident optical system for guiding the
plurality of light fluxes emitted from the plurality of light emitting
portions to the deflection unit; andan imaging optical system for forming
images of the plurality of light fluxes deflected for scanning by a
deflection surface of the deflection unit on a surface to be scanned,
wherein:the incident optical system includes an optical element including
at least one noncircular optical surface in a main scanning section, and
an aperture stop for restricting a light flux width at least in the main
scanning direction of the light flux entering the deflection unit, which
are disposed in the stated order from a side of the light source unit;the
noncircular optical surface in the main scanning section of the optical
element has a shape in which a positive power decreases from on axis
toward off axis; andwhen a space between light emitting portions that are
located farthest from an optical axis in the main scanning direction
among the plurality of light emitting portions of the light source unit
is denoted by W (mm), an optical path length between the aperture stop
and an optical surface that is closest to the light source unit among the
at least one noncircular optical surface of the incident optical system
in the main scanning section is denoted by La (mm), a focal length of the
incident optical system in the main scanning direction is denoted by
f1 (mm), and a light flux width in the main scanning direction of
the light flux that is emitted from the light emitting portion which is
located farthest from the optical axis in the main scanning direction on
the optical surface closest to the light source unit among the at least
one noncircular optical surface in the main scanning section of the
incident optical system is denoted by D (mm), the following expression is
satisfied2D≧|WLa/2f1|≧D/8.

2. An optical scanning apparatus according to claim 1, wherein the
following expression is further
satisfied8.times.f1>|La|>2.times.f.sub.1.

3. An optical scanning apparatus according to claim 1, wherein the
incident optical system includes a first optical element, a second
optical element having a power in a sub scanning direction for forming
images of the plurality of light fluxes that have passed through the
first optical element on the deflection surface of the deflection unit as
linear images elongated in the main scanning direction, and the aperture
stop, which are disposed in the stated order from the side of the light
source unit.

4. An optical scanning apparatus according to claim 3, wherein the optical
element including the at least one noncircular optical surface in the
main scanning section is the first optical element.

5. An optical scanning apparatus according to claim 3, wherein the optical
element including the at least one noncircular optical surface in the
main scanning section is the second optical element.

6. An optical scanning apparatus according to claim 1, wherein, when an
optical path length between the light source unit and the deflection
surface of the deflection unit is denoted by L (mm), an optical path
length between the aperture stop and the deflection surface of the
deflection unit is denoted by M (mm), the following expression is
satisfied0<M/L<0.6.

7. An optical scanning apparatus according to any one of claims 1 to 6,
wherein the plurality of light emitting portions includes four or more
light emitting portions that are spaced from one another in the main
scanning direction.

8. An image forming apparatus, comprising:the optical scanning apparatus
according to claim 1;a photosensitive member disposed on the surface to
be scanned;a developing device for developing an electrostatic latent
image as a toner image that is formed on the photosensitive member with a
light beam deflected for scanning by the optical scanning apparatus;a
transferring device for transferring the developed toner image to a
transfer material; anda fixing device for fixing the transferred toner
image on the transfer material.

9. An image forming apparatus, comprising:the optical scanning apparatus
according to claim 1; anda printer controller for converting code data
supplied from an external device into an image signal, which is supplied
to the optical scanning apparatus.

Description:

BACKGROUND OF THE INVENTION

[0001]1. Field of the Invention

[0002]The present invention relates to an optical scanning apparatus and
an image forming apparatus using the same. The present invention is
suited to an image forming apparatus such as a laser beam printer, a
digital copying machine, or a multi-function printer, which adopts an
electrophotography process.

[0003]2. Description of the Related Art

[0004]Conventionally, in an optical scanning apparatus that is used for a
laser beam printer or a digital copying machine, a light flux emitted
from a light source unit is guided to a light deflecting device by an
incident optical system.

[0005]In such optical scanning apparatus, high speed and high resolution
can be achieved by increasing the number of light emitting portions of
the light source unit.

[0006]There are conventionally proposed various optical scanning
apparatuses that can achieve high speed and high resolution by increasing
the number of light emitting portions of the light source unit (see
Japanese Patent Application Laid-Open No. H09-26550, and Japanese Patent
Application Laid-Open No. 2001-154128).

[0007]Japanese Patent Application Laid-Open No. H09-26550 discloses a
technology for improving optical performance among a plurality of light
fluxes by arranging a plurality of light emitting portions in a symmetric
manner with respect to an optical axis of a collimator lens.

[0008]Japanese Patent Application Laid-Open No. 2001-154128 discloses a
technology of adjusting a light source and a laser for improving optical
performance among a plurality of light fluxes emitted from a plurality of
light emitting portions on a surface to be scanned.

[0009]In the conventional optical scanning apparatuses described above, if
a multibeam light source unit including a plurality of light emitting
portions at positions far from the optical axis of an incident optical
system in a main scanning direction is used for a light source unit, a
focal position on the surface to be scanned of each light flux emitted
from each light emitting portion of the multibeam light source unit is
shifted so that a difference of spot diameter occurs between the light
fluxes. Thus, there is a problem of deterioration of an image.

[0010]In addition, it is necessary to increase the number of lenses of the
incident optical system so that the difference of the spot diameter
between light fluxes does not occur. This causes not only problems of
upsizing of the entire apparatus and complication thereof but also
problems that sensitivity of the incident optical system is increased,
and hence a performance deterioration due to a manufacturing error is
increased.

[0011]It is supposed to use a light source unit including two light
emitting portions having different distances from the optical axis of the
collimator lens in the main scanning direction (due to designing and
manufacturing errors) as the light source unit including a plurality of
light emitting portions that are spaced apart from one another in the
main scanning direction.

[0012]In this case, if a shape of a lens surface of the collimator lens
forming the incident optical system is circular in the main scanning
section, a field curvature in the main scanning direction occurs in the
lens surface of the collimator lens.

[0013]In other words, a difference of condensing state in the main
scanning direction occurs between two light fluxes that have passed
through the lens surface of the collimator lens.

[0014]For example, a difference of parallelism in the main scanning
direction occurs between two collimated light fluxes that have passed
through the lens surface of the collimator lens.

[0015]The focal positions of the two light fluxes emitted from the two
light emitting portions on the surface to be scanned differ from each
other. As a result, the spot diameters of the two light fluxes on the
surface to be scanned are different from each other, and this causes an
image quality difference between images based on the two light fluxes
emitted from the two light emitting portions.

SUMMARY OF THE INVENTION

[0016]It is an object of the present invention to provide an optical
scanning apparatus and an image forming apparatus using the same, which
are capable of reducing a field curvature in a main scanning direction,
which occurs when a plurality of light fluxes having been emitted from a
plurality of light emitting portions that are spaced apart from one
another in the main scanning direction pass through an incident optical
system.

For achieving the above described object, one aspect of the present
invention is an optical scanning apparatus, comprising: a light source
unit including a plurality of light emitting portions that are spaced
apart from one another in a main scanning direction; a deflection unit
for deflecting a plurality of light fluxes emitted from the plurality of
light emitting portions for scanning; an incident optical system for
guiding the plurality of light fluxes emitted from the plurality of light
emitting portions to the deflection unit; and an imaging optical system
for forming images of the plurality of light fluxes deflected for
scanning by a deflection surface of the deflection unit on a surface to
be scanned, wherein: the incident optical system includes an optical
element including at least one noncircular optical surface in a main
scanning section, and an aperture stop for restricting a light flux width
at least in the main scanning direction of the light flux entering the
deflection unit, which are disposed in the stated order from a side of
the light source unit; the noncircular optical surface in the main
scanning section of the optical element has a shape in which a positive
power decreases from on axis toward off axis; and when a space between
light emitting portions that are located farthest from an optical axis in
the main scanning direction among the plurality of light emitting
portions of the light source unit is denoted by W (mm), an optical path
length between the aperture stop and an optical surface that is closest
to the light source unit among the at least one noncircular optical
surface of the incident optical system in the main scanning section is
denoted by La (mm), a focal length of the incident optical system in the
main scanning direction is denoted by f1 (mm), and a light flux
width in the main scanning direction of the light flux that is emitted
from the light emitting portion which is located farthest from the
optical axis in the main scanning direction on the optical surface
closest to the light source unit among the at least one noncircular
optical surface in the main scanning section of the incident optical
system is denoted by D (mm), the following expression is satisfied

2D≧|WLa/2f1|≧D/8.

[0017]In the optical scanning apparatus as described above, it is
preferable that the following expression is further satisfied

8×f1>|La|>2×f1.

[0018]In addition, in the optical scanning apparatus as described above,
it is preferable that the incident optical system includes a first
optical element, a second optical element having a power in a sub
scanning direction for forming images of the plurality of light fluxes
that have passed through the first optical element on the deflection
surface of the deflection unit as linear images elongated in the main
scanning direction, and the aperture stop, which are disposed in the
stated order from the side of the light source unit.

[0019]In such optical scanning apparatus, it is further preferable that
the optical element including the at least one noncircular optical
surface in the main scanning section is the first optical element.

[0020]Alternatively, it is also preferable that the optical element
including the at least one noncircular optical surface in the main
scanning section is the second optical element.

[0021]Furthermore, the optical scanning apparatus as described above, it
is preferable that when an optical path length between the light source
unit and the deflection surface of the deflection unit is denoted by L
(mm), an optical path length between the aperture stop and the deflection
surface of the deflection unit is denoted by M (mm), the following
expression is satisfied

0<M/L<0.6.

[0022]In addition, it is preferable that the plurality of light emitting
portions includes four or more light emitting portions that are spaced
from one another in the main scanning direction.

[0023]Furthermore, for achieving the above described object, one aspect of
the present invention is an image forming apparatus, comprising: the
optical scanning apparatus as described above; a photosensitive member
disposed on the surface to be scanned; a developing device for developing
an electrostatic latent image as a toner image that is formed on the
photosensitive member with a light beam deflected for scanning by the
optical scanning apparatus; a transferring device for transferring the
developed toner image to a transfer material; and a fixing device for
fixing the transferred toner image on the transfer material.

[0024]Alternatively, further aspect of the present invention is an image
forming apparatus, comprising: the optical scanning apparatus as
described above; and a printer controller for converting code data
supplied from an external device into an image signal, which is supplied
to the optical scanning apparatus.

[0025]According to the present invention, the field curvature in the main
scanning direction can be reduced, which occurs when the plurality of
light fluxes having been emitted from the plurality of light emitting
portions that are spaced apart from one another in the main scanning
direction pass through the incident optical system.

[0026]As a result, a variation in spot diameter on the surface to be
scanned, of the plurality of light fluxes emitted from the plurality of
light emitting portions that are spaced apart from one another in the
main scanning direction, can be reduced.

[0027]Further features of the present invention will become apparent from
the following description of exemplary embodiments with reference to the
attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0028]FIG. 1 illustrates a main scanning section according to a first
embodiment of the present invention.

[0029]FIG. 2 illustrates a sub scanning section according to the first
embodiment of the present invention.

[0030]FIG. 3 illustrates a main scanning section of an incident optical
system according to the first embodiment of the present invention.

[0031]FIG. 4 illustrates a field curvature of the incident optical system
according to the first embodiment of the present invention.

[0032]FIG. 5 is a schematic diagram of a light source unit according to
the first embodiment of the present invention.

[0033]FIG. 6 is an enlarged view of the light source unit and a collimator
lens according to the first embodiment of the present invention.

[0034]FIG. 7 illustrates a main scanning section of an incident optical
system according to a second embodiment of the present invention.

[0035]FIG. 8 illustrates a field curvature in a main scanning direction of
the incident optical system according to the second embodiment of the
present invention.

[0036]FIG. 9 illustrates a position of a light flux that passes through
the collimator lens according to the first embodiment of the present
invention.

[0037]FIG. 10 illustrates a main scanning section of an incident optical
system according to a third embodiment of the present invention.

[0038]FIG. 11 illustrates a field curvature in a main scanning direction
of the incident optical system according to the third embodiment of the
present invention.

[0039]FIG. 12 is a schematic diagram of a light source unit according to
the third embodiment of the present invention.

[0040]FIG. 13 is a schematic diagram of a color image forming apparatus
according to the present invention.

DESCRIPTION OF THE EMBODIMENTS

First Embodiment

[0041]FIG. 1 is a main portion sectional view of a main scanning direction
(main scanning sectional view) according to a first embodiment of the
present invention. FIG. 2 is a main portion sectional view of a sub
scanning direction (sub scanning sectional view) according to the first
embodiment of the present invention.

[0042]It should be noted that, in the description below, the sub scanning
direction (Z direction) is a direction parallel to a rotational axis of a
deflection unit. A main scanning section is a section in which the sub
scanning direction (direction parallel to the rotational axis of the
deflection unit) is a normal line. A main scanning direction (Y
direction) is a direction in which light fluxes deflected for scanning by
a deflection surface of the deflection unit are projected on the main
scanning section. A sub scanning section is a section in which the main
scanning direction is a normal line.

[0043]In the diagram, a light source unit 1 includes a semiconductor laser
including a plurality of light emitting portions that are spaced apart
from one another in the main scanning direction and in the sub scanning
direction. One of the plurality of light emitting portions of the
multibeam semiconductor laser 1 has a distance from the optical axis of a
collimator lens 3 in the main scanning direction, which is different from
a distance from the optical axis, of another one of the plurality of the
light emitting portions. The semiconductor laser 1 includes an 8-beam
laser in which eight light emitting portions are arranged in
one-dimensional manner so as to be spaced apart from one another in the
main scanning direction and in the sub scanning direction as illustrated
in FIG. 5.

[0044]The first aperture stop (sub scanning stop) 2 restricts the light
flux width of the passing light flux in the sub scanning direction so as
to shape the beam shape. The first optical element 3 converts the light
flux emitted from the light source unit 1 into a collimated light flux,
and includes a glass mold lens manufactured by a molding process.

[0045]The collimator lens 3 serving as the first optical element has an
exit surface that corresponds to a lens surface (optical surface) having
a rotational symmetrical noncircular surface, in which a convex
(positive) power decreases from the lens optical axis toward the
peripheral portion.

[0046]Thus, focal positions of light fluxes emitted from the plurality of
light emitting portions on the surface to be scanned become the same so
that spot diameters of the plurality of light fluxes on the surface to be
scanned become the same. In addition, the diverged light flux emitted
from the light source unit 1 is converted into a collimated light flux in
the main scanning section and in the sub scanning section.

[0047]Note that the exit surface of the collimator lens 3 is the
noncircular surface in the main scanning section in this embodiment, but
this structure should not be interpreted as a limitation. The incident
surface or both the surfaces may be the noncircular surface in the main
scanning section.

[0048]A cylindrical lens 4 serves as a second optical element 4 that has a
power only in the sub scanning direction and is manufactured by the
molding process. The cylindrical lens 4 works so that the light flux
having passed through the collimator lens 3 forms, in the sub scanning
section, an image elongated in the main scanning direction on a
deflection surface 10a of a light deflecting device 10.

[0049]It should be noted that the collimator lens 3 and the cylindrical
lens 4 may be formed of an anamorphic lens serving as one combined
optical element. The anamorphic lens has both a collimate function of
making different from each other a power in the main scanning direction
and a power in the sub scanning direction, and a function of forming an
image on the deflection surface in the sub scanning direction.

[0051]In addition, the second aperture stop 5 makes the main light beams
of the light fluxes from the individual light emitting portions be close
to each other on the deflection surface 10a, to thereby reduce a jitter
amount in the main scanning direction on the surface to be scanned, of
the plurality of light fluxes emitted from the plurality of light
emitting portions that are spaced apart from one another in the main
scanning direction. The jitter amount in the main scanning direction
indicates an imaging position difference in the main scanning direction
among a plurality of spots imaged on the surface to be scanned.

[0052]Note that each of the first aperture stop 2, the collimator lens 3,
the cylindrical lens 4, and the second aperture stop 5 constitutes an
element of the incident optical system LA. The light deflecting device 10
includes a polygon mirror (rotational polygon mirror) having five
surfaces, and is rotated at a constant speed in the direction of the
arrow A in the diagram by a driving unit such as a motor (not shown). An
imaging optical system 6 having a condensing function and fθ
characteristics includes a first imaging lens 6a and a second imaging
lens 6b.

[0053]The first imaging lens 6a and the second imaging lens 6b both
include an anamorphic lens having a noncircular surface shape in the main
scanning section. The imaging optical system 6 causes the light flux
based on image information deflected for scanning by the deflection
surface of the light deflecting device 10 to form an image on a
photosensitive drum surface 7 as the surface to be scanned. Further, the
imaging optical system 6 has a function as a face tilt compensating
optical system for setting the deflection surface 10a of the light
deflecting device 10 and the photosensitive drum surface 7 to be
conjugate to each other in the sub scanning section. The photosensitive
drum surface 7 serves as the surface to be scanned.

[0054]Eight light fluxes 1a, 1b, 1c . . . that are optically modulated in
accordance with the image information and emitted from the light source
unit 1 are restricted by the first aperture stop 2 concerning the light
flux width in the sub scanning direction. Then, the eight light fluxes
are converted by the collimator lens 3 into collimated light fluxes,
which enter the cylindrical lens 4. The light flux having entered the
cylindrical lens 4 exits with its condensing state not being changed in
the main scanning section, and the light flux width thereof in the main
scanning direction is restricted by the second aperture stop 5.

[0055]In addition, the light flux having entered the cylindrical lens 4
converges in the sub scanning section and the light flux width thereof in
the main scanning direction is restricted by the second aperture stop 5,
so as to form a linear image (linear image elongated in the main scanning
direction) on the deflection surface 10a of the light deflecting device
10.

[0056]Then, each of the plurality of light fluxes deflected for scanning
by the deflection surface 10a of the light deflecting device 10 forms a
spot image by the imaging optical system 6 on the photosensitive drum
surface 7.

[0057]Further, when the light deflecting device 10 is rotated in the
direction of the arrow A, the photosensitive drum surface 7 is scanned
with each of the plurality of light fluxes deflected for scanning by the
deflection surface 10a of the light deflecting device 10 in the direction
of the arrow B (in the main scanning direction) at a constant speed.
Thus, a plurality of scanning lines are formed simultaneously for
recording the image on the photosensitive drum surface 7 serving as a
recording medium.

[0058]As illustrated in FIG. 2, three flat surface mirrors 8a, 8b, and 8c
are disposed in the optical path from the light deflecting device 10 to
the surface to be scanned 7. Thus, the optical path of the imaging
optical system 6 can be folded in compact size, and hence the entire
apparatus can be downsized.

[0059]The semiconductor laser 1 as the light source unit includes eight
light emitting portions arranged in the one-dimensional direction at a
pitch distance of 50 μm with a tilt angle α (α=9.2
degrees) from the main scanning direction as illustrated in FIG. 5.

[0060]In addition, the semiconductor laser 1 is supported by the incident
optical system LA in a rotatable manner about the axis parallel to the
optical axis, in order to adjust a beam spacing error due to an
attachment error when the semiconductor laser 1 is assembled.

[0061]Note that FIG. 5 illustrates an off axis light emitting portion 1a
that is closest to the axis, and off axis light emitting portions 1b and
1c farthest in the main scanning direction. The light emitting portions
1b and 1c are arranged in the main scanning direction in a symmetric
manner with respect to the optical axis of the incident optical system
LA.

[0062]W (mm) denotes a space between the light emitting portions 1b and
1c, which are farthest from the optical axis in the main scanning
direction among the plurality of light emitting portions of the light
source unit 1.

[0063]Note that the semiconductor laser including eight light emitting
portions is used in this embodiment, but the present invention can be
applied to other semiconductor laser including two or more light emitting
portions that are disposed at different positions from each other with
respect to the optical axis in the main scanning direction.

[0064]However, the problem to be solved by the present invention is
inherent especially in a multibeam optical scanning apparatus having a
larger distance from the optical axis to the light emitting portions in
the main scanning direction.

[0065]Therefore, the present invention produces more effect when applied
to a multibeam optical scanning apparatus including four or more light
emitting portions that are disposed at different positions from one
another with respect to the optical axis in the main scanning direction.

[0066]In terms of design, the off axis light emitting portion 1a that is
closest to the axis and the off axis light emitting portions 1b and 1c
farthest in the main scanning direction are disposed at different
positions from one another with respect to the optical axis in the main
scanning direction.

[0067]Accordingly, when the shape of the lens surface of the collimator
lens 3 is circular in the main scanning section, a field curvature in the
main scanning direction occurs in the lens surface of the collimator lens
3 with respect to a light flux a emitted from the off axis light emitting
portion 1a that is closest to the axis and light fluxes b and c emitted
respectively from the off axis light emitting portions 1b and 1c farthest
in the main scanning direction. Note that the reference symbols a, b, and
c of the light fluxes are added to distinguish the above-mentioned light
fluxes emitted from the light emitting portions 1a, 1b, and 1c from one
another in the description, and the reference symbols a, b, and c are not
shown in the drawings.

[0068]In other words, a difference of condensing state in the main
scanning direction occurs between the light flux a and the light fluxes b
and c that have passed through the lens surface of the collimator lens.

[0069]A difference of parallelism in the main scanning direction occurs
between the collimated light flux a and the collimated light fluxes b and
c that have passed through the lens surface of the collimator lens.

[0070]As a result, focal positions of the light flux a and the light
fluxes b and c on the surface to be scanned differ from each other, and
thus there arises a problem that spot diameters of the light flux a and
the light fluxes b and c on the surface to be scanned differ from each
other.

[0071]Similarly, there arises a problem that the spot diameters of the
light flux b and the light flux c on the surface to be scanned, which are
emitted respectively from the off axis light emitting portions 1b and 1c
farthest in the main scanning direction, which are arranged in the main
scanning direction in a symmetric manner with respect to the optical axis
of the incident optical system LA, differ from each other due to
designing arrangement errors.

[0072]Optical parameters used in the incident optical system according to
this embodiment are set as in Table 1.

[0073]In this embodiment, in the main scanning section, the exit surface
of the collimator lens 3 is formed to be noncircular in which a positive
power decreases from the axis toward the off axis, to thereby compensate
for the field curvature on the surface to be scanned in the main scanning
direction.

[0074]In addition, the collimator lens 3 has an incident surface provided
with a weak convex power (r=100 mm) so as to be advantageous in molding
the lens.

[0075]An expression for defining the lens shape of the collimator lens 3
is as follows.

[0076]In addition, in the imaging optical system, the intersection of the
imaging lens and the optical axis is regarded as the origin. As
illustrated in FIG. 1, at the scanning start side and the scanning end
side with respect to the optical axis of the imaging optical system, the
optical axis is regarded as the X-axis, the direction perpendicular to
the optical axis in the main scanning section is regarded as the Y-axis,
and the direction perpendicular to the optical axis in the sub scanning
section is regarded as the Z-axis. Then, the imaging optical system can
be expressed by the following functions.

[0078]In this embodiment, the shapes in the main scanning section, of the
first imaging lens 6a and the second imaging lens 6b are formed
symmetrically with respect to the optical axis.

[0079]In other words, aspheric surface coefficients of the first imaging
lens 6a and the second imaging lens 6b on the scanning start side and on
the scanning end side are made to be the same.

[0080]Each of the incident surface and the exit surface of the first
imaging lens 6a, and the exit surface of the second imaging lens 6b is
the noncircular surface in the main scanning section.

[0081]The incident surface of the second imaging lens 6b is the circular
surface in the main scanning section.

[0082]Further, the curvature radii in the sub scanning direction of the
incident surface R1 and the exit surface R2 of the first imaging lens 6a
are constant without varying between the axis and the off axis.

[0083]The curvature radii in the sub scanning direction of the incident
surface R3 and the exit surface R4 of the second imaging lens 6b vary
between the axis and the off axis in an asymmetric manner.

[0084]With respect to the optical axis, on the scanning start side and the
scanning end side, the optical axis is regarded as the X-axis, the
direction perpendicular to the optical axis in the main scanning section
is regarded as the Y-axis, and the direction perpendicular to the optical
axis in the sub scanning section is regarded as the Z-axis. Then, the
shape in the sub scanning section can be expressed by the following
continuous functions.

[0085]The following continuous functions are functions that define the
shapes in the sub scanning direction, of R1, R2, R3, and R4 surfaces.

[0086]where r' denotes the curvature radius in the sub scanning direction,
D2, D4, D6, D8, and D10 denote coefficients, s
denotes a suffix of the coefficient of the scanning start side, and e
denotes the scanning end side.

[0087]The curvature radius in the sub scanning direction corresponds to a
curvature radius in the cross section orthogonal to the generatrix in the
main scanning direction. In other words, the curvature radius in the sub
scanning direction corresponds to a curvature radius in the cross section
including a normal line on the generatrix of the lens surface.

[0088]The imaging optical system includes two imaging lenses in this
embodiment, but this structure should not be interpreted to be a
limitation. The imaging optical system may include one or three or more
imaging optical elements.

[0089]Table 2 illustrates values of the optical scanning apparatus
according to the first embodiment of the present invention. Here, "E-x"
means "10-x".

[0090]R1 surface is an incident surface of the first imaging lens 6a on
the side of the light deflecting device 10. R2 surface is an exit surface
of the first imaging lens 6a on the side of the surface to be scanned 7.
R3 surface is an incident surface of the second imaging lens 6b on the
side of the light deflecting device 10. R4 surface is an exit surface of
the second imaging lens 6b on the side of the surface to be scanned 7.

[0091]Table 2 described below illustrates optical parameter values used in
the imaging optical system of this embodiment.

[0092]In this embodiment, the light emitting portion spacing in the main
scanning direction, of the light emitting portion 1b and the light
emitting portion 1c, which are farthest from the optical axis among the
plurality of light emitting portions of the light source unit 1, is
denoted by W (mm).

[0093]An incident surface of the collimator lens 3 is a spherical surface
that is rotationally symmetrical to the optical axis. An exit surface of
the collimator lens 3 is an aspheric surface that is rotationally
symmetrical to the optical axis. The collimator lens 3 has the same focal
length in the main scanning direction and in the sub scanning direction.

[0094]Further, a light flux width in the main scanning direction, of the
light flux emitted from the light emitting portion, which is farthest
from the optical axis in the main scanning direction, on the optical
surface closest to the light source unit 1 among the noncircular optical
surfaces of the incident optical system LA in the sub scanning direction
is denoted by D (mm).

[0095]The light flux width D in this embodiment is a light flux width in
the main scanning direction, of the light flux b emitted from the light
emitting portion 1b, which is farthest from the optical axis in the main
scanning direction, on the exit surface of the collimator lens 3, and is
restricted by the aperture width of the second aperture stop 5.

[0096]Now, referring to FIG. 9 illustrating the main scanning section (X-Y
section) of the incident optical system, calculation of a passage
position X of the light flux b in the collimator lens 3 which has been
emitted from the off axis light emitting portion 1b (FIG. 5) in the main
scanning direction is described.

[0097]A passage position of the light flux c emitted from the off axis
light emitting portion 1c (FIG. 5) can be calculated in a similar manner
to the off axis light emitting portion 1b, and hence calculation of the
passage position of the light flux c is omitted here.

[0098]Each optical element illustrated in FIG. 9 is denoted by the same
numeral as used for the same optical element illustrated in FIG. 1 and
FIG. 5.

[0099]FIG. 9 illustrates an entrance pupil position (conjugate position of
the main scanning aperture stop with respect to the collimator lens 3)
5a, and a rear principal point 3a and a front principal point 3b of the
collimator lens 3.

[0100]Further, f1 (mm) denotes a focal length of the collimator lens
3, La (mm) denotes an optical path length from the rear principal point
3a to the second aperture stop 5, and Lb (mm) denotes an optical path
length from the front principal point 3b to the entrance pupil position
5a.

[0101]A principal ray of the light flux b having been emitted from the off
axis light emitting portion 1b of the light source unit 1 passes through
the collimator lens 3 at a position that is x away from the optical axis
of the collimator lens 3, and intersects with the optical axis of the
collimator lens 3 at the second aperture stop 5. Accordingly, Expression
A is obtained through paraxial calculation.

1 Lb = 1 La + 1 f 1 Expression A ##EQU00004##

[0102]Then, Expression B is obtained through geometric calculation from
the similarity of triangles.

Lb x = Lb - f 1 W / 2 Expression B
##EQU00005##

[0103]Expression C is obtained by substituting Expression A into
Expression B.

x = W × La 2 f 1 Expression C
##EQU00006##

[0104]In the derivation for Expression C, the influence of refraction by
the cylindrical lens 4 is neglected because the cylindrical lens 4 is the
optical element having no power in the main scanning direction.

[0105]Further, the optical path length from the light source unit 1 to the
front principal point of the collimator lens 3 slightly deviates from f
because of the influence of wave aberration, but the deviation is
neglected.

[0106]Therefore, in the present invention, the optical path length La from
the rear principal point 3a to the second aperture stop 5 is considered
to approximate the optical path length from the noncircular exit surface
of the collimator lens 3 in the main scanning section to the second
aperture stop 5.

[0107]Accordingly, the space between the light emitting portions 1b and
1c, which are farthest from the optical axis in the main scanning
direction among the plurality of light emitting portions of the light
source unit 1, is denoted by W (mm), and the optical path length between
the above-mentioned aperture stop and the optical surface that is closest
to the light source unit 1 among the noncircular optical surfaces of the
incident optical system LA in the main scanning section is denoted by La
(mm).

[0108]The focal length of the incident optical system LA in the main
scanning direction is denoted by f1 (mm), and the light flux width
in the main scanning direction, of each of the light fluxes emitted from
the light emitting portions 1b and 1c, which are farthest from the
optical axis in the main scanning direction, on the optical surface
closest to the light source unit 1 among the noncircular optical surfaces
of the incident optical system LA in the main scanning section is denoted
by D (mm).

[0109]In this case, the individual elements are set so as to satisfy the
following expression.

2D≧|WLa/2f1|≧D/8 (1)

[0110]Over the upper limit value of the conditional expression (1), an
external shape of the collimator lens 3 is increased in the main scanning
direction, leading to increase in size of the incident optical system,
which is not appropriate.

[0111]Further, under the lower limit value of the conditional expression
(1), on the noncircular lens surface in the main scanning section of the
collimator lens 3, the amount x by which the principal ray of the light
flux b emitted from the off axis light emitting portion 1b, which is
farthest from the optical axis of the collimator lens 3, is away from the
optical axis is reduced.

[0112]In this case, when the out-of-parallelism in the main scanning
section occurring in the lens surface of the collimator lens 3 is to be
compensated by forming the shape of the exit surface of the collimator
lens 3 in the main scanning section into the noncircular shape in which a
positive power continuously decreases from the axis toward the off axis,
the effect of the aspheric surface cannot be used effectively.

[0113]The reason is as follows. The aspheric surface coefficients A, B, C,
. . . are coefficients of the h4 term, the h6 term, the h8
term, . . . , respectively, as expressed in Equation 1, and hence the
effect of the aspheric surface can be used effectively more when the
aspheric surface is applied with respect to the light flux passing
through a position that is away from the optical axis than applied with
respect to the light flux passing through the vicinity of the optical
axis of the collimator lens 3.

[0114]h is set considering an amount x by which a light flux is away from
the optical axis of the collimator lens 3. As a precondition, the
aspheric surface coefficients A, B, C, . . . are determined taking into
consideration the balance with respect to all of the optical parameters
used in the optical scanning apparatus, which contribute to optical
performances of the plurality of light fluxes on the surface to be
scanned.

[0115]Therefore, the aspheric surface coefficients A, B, C, . . . cannot
be dealt as design values for solving only the problem of the present
invention.

[0116]In general, the multibeam optical scanning apparatus is designed
such that a virtual light flux having been emitted from a virtual light
emitting portion, which is arranged on the optical axis of the collimator
lens 3, is converted into a completely collimated light flux by the
collimator lens 3.

[0117]Accordingly, design is made such that the out-of-parallelism in the
main scanning section occurring in the lens surface of the collimator
lens 3 is reduced in the collimated light flux a emitted from the off
axis light emitting portion 1a at a vicinity of the optical axis of the
collimator lens 3 than in the collimated light flux b emitted from the
off axis light emitting portion 1b, which is farthest from the optical
axis of the collimator lens 3.

[0118]The off axis light emitting portion 1a is disposed at a vicinity of
the optical axis of the collimator lens 3, and hence the amount x by
which the light flux a is away from the optical axis at a time when the
light flux a passes through the exit surface of the collimator lens 3,
which is formed to be noncircular in the main scanning section, is
reduced.

[0119]However, in the case of the light flux a emitted from the off axis
light emitting portion 1a, the out-of-parallelism in the main scanning
section occurring in the lens surface of the collimator lens 3 is small,
and hence no problem arises when a compensation amount therefor by the
effect of the aspheric surface is small.

[0120]The technical meaning of the conditional expression (1) is described
below.

[0121]By making larger the value of |WLa/2f| in the conditional expression
(1), the space amount x between the optical axis and the principal ray of
the light flux b emitted from the off axis light emitting portion 1b,
which is farthest from the optical axis of the collimator lens 3, can be
increased.

[0122]Accordingly, the effect of the aspheric surface produced by forming
the shape of the exit surface of the collimator lens 3 in the main
scanning section into the noncircular shape in which the positive power
continuously decreases from the axis toward the off axis can be used
effectively.

[0123]To make larger the value of |WLa/2f1|, it is only necessary to
make larger the values of |La| and W, or smaller the value of f1.

[0124]However, the value of W is restricted by a scanning line pitch on
the surface to be scanned.

[0125]Accordingly, in this embodiment, by satisfying
"|La|>2×f1", the space amount x between the optical axis
and the main light beam of the light flux b emitted from the off axis
light emitting portion 1b, which is farthest from the optical axis of the
collimator lens 3, is increased.

[0126]In this embodiment, in order to increase the space amount x,
"|La|>80 mm" can be appropriately set.

[0127]In order to prevent the optical path length of the incident optical
system from being large to increase the incident optical system in size,
"8×f1>|La|" and "200 mm>|La|" are preferred to be set.

[0128]Accordingly, the individual elements are set so as to satisfy the
following expression.

8×f1>|La|>2×f1 (2)

[0129]In this embodiment, it is preferred that the value of W satisfy
"0.25<W<1".

[0130]Under the lower limit value of W, there arises a problem of
crosstalk between light fluxes emitted from adjacent light emitting
portions. Over the upper limit value of W, there arises a problem that
the optical system of the optical scanning apparatus has a low degree of
design freedom when the scanning line pitch (resolution) on the surface
to be scanned is attained.

[0131]Taking into consideration a size of the spot diameter on the surface
to be scanned in the main scanning direction, in this embodiment, it is
preferred that the value of D satisfy "2<D<8".

[0132]In addition, the optical path length from the light source unit 1 to
the deflection surface 10a of the light deflecting device 10 is denoted
by L (mm), and the optical path length from the second aperture stop 5 to
the deflection surface 10a of the light deflecting device 10 is denoted
by M (mm). Here, the optical path length to the deflection surface
corresponds to an optical path length to the point where a central light
flux of the light flux enters the deflection surface when the deflection
surface scans the center of the scanning range on the surface to be
scanned.

[0133]In order to reduce the jitter amount in the main scanning direction
on the surface to be scanned, of the plurality of light fluxes emitted
from the plurality of light emitting portions that are spaced from one
another in the main scanning direction, the individual elements are set
so as to satisfy the following conditional expression.

0<M/L<0.6 (3)

[0134]When the value of M is smaller, the second aperture stop 5 can be
made closer to the light deflecting device 10, to thereby reduce the
jitter amount in the main scanning direction.

[0135]It is preferred to satisfy 0<M<50 and 100<L<300.

[0136]Further, the conditional expressions (1), (2), and (3) are preferred
to be set as follows.

D≧|WLa/2f1|≧D/6 (1a)

|La|>2.5×f1 (2a)

0<M/L<0.5 (3a)

[0137]Further, in this embodiment, the semiconductor laser 1 as the light
source unit includes an 8-beam laser of 50 μm pitch and is inclined by
9.2 degrees with respect to the main scanning direction. The plurality of
light emitting portions are aligned in the main scanning direction in the
one-dimensional manner, and hence an optical zooming factor in the sub
scanning direction can be increased. Therefore, the light emitting
portion spacing of the semiconductor lasers can be increased.

[0138]FIG. 3 illustrates a schematic diagram of the incident optical
system from the light source unit 1 to the light deflecting device 10
illustrated in FIG. 1. In FIG. 3, the element that is the same as the
element illustrated in FIG. 1 is denoted by the same reference symbol.
Among the above-mentioned eight light emitting portions (light emitting
points) illustrated in FIG. 5, the light emitting portion 1b disposed at
the position farthest from the optical axis of the collimator lens 3 in
the main scanning direction is at the position 0.175 mm away in the main
scanning direction.

[0139]FIG. 4 illustrates the field curvature in the main scanning
direction and in the sub scanning direction, which occurs in the
collimator lens 3.

[0140]In FIG. 4, the field curvature shape in the main scanning direction
is the same as the field curvature shape in the sub scanning direction.

[0141]In FIG. 4, the vertical axis represents a position of the light
emitting portion in the main scanning direction, and the horizontal axis
represents a field curvature amount of the collimator lens 3. The
coordinate position 400 (horizontal axis=0) indicates a focal position of
the off axis light emitting portion 1a at a vicinity of the optical axis,
and the coordinate position 401 (horizontal axis=0.17) indicates the
field curvature of the farthest off axis light emitting portion 1b.

[0142]As understood from FIG. 4, the off axis light emitting portion 1a
(400 in FIG. 4) at a vicinity of the optical axis and the off axis light
emitting portion 1b (401 in FIG. 4) farthest in the main scanning
direction have different coordinates in the horizontal axis.

[0143]In other words, the light fluxes from the off axis light emitting
portion 1a at a vicinity of the optical axis and the farthest off axis
light emitting portion 1b form images at positions shifted from each
other in the focusing direction in the main scanning direction (X-axis
direction) on the surface to be scanned.

[0144]The light emitting portion 1a at a vicinity of the optical axis has
a zero design focus shift in the main scanning direction on the surface
to be scanned, and the focal position of the farthest off axis light
emitting portion 1b in the main scanning direction on the surface to be
scanned is shifted by ΔM expressed by the following equation.

ΔM=Δmcol|×(ffθ/f1)2

[0145]where:

|Δmcol| denotes a field curvature difference between the light
emitting portion 1a and the light emitting portion 1b in the main
scanning direction of the incident optical system LA;ffθ
denotes a focal length of the imaging optical system 6 in the main
scanning section; andf1 denotes a focal length of the collimator
lens 3.

[0146]In addition, as illustrated in FIG. 4, the incident optical system
LA of this embodiment has a structure in which the field curvature is
well compensated even if the light emitting portion is at a position 0.5
mm away from the optical axis, and the focal difference between the light
emitting portions in the main scanning direction on the surface to be
scanned can be suppressed even if a semiconductor laser having a space of
W=1.0 mm between the light emitting portions that are farthest from each
other in the main scanning direction is used.

[0147]FIG. 6 illustrates an enlarged view of the collimator lens 3 at a
vicinity of the light source unit 1. In FIG. 6, the element that is the
same as the element illustrated in FIG. 1 is denoted by the same
reference symbol.

[0148]The light fluxes from the off axis light emitting portion 1a at a
vicinity of the optical axis and the off axis light emitting portion 1b
farthest in the main scanning direction are emitted from the cylindrical
lens 4 illustrated in FIG. 3 with substantially the same angle of
divergence. This indicates that the plurality of light fluxes emitted
from the collimator lens 3 have no field curvature in the main scanning
direction. If the collimator lens 3 has a field curvature in the main
scanning direction, a difference between the angles of divergence occurs.

[0149]In general, in order to reduce the field curvature, the following
values should be set appropriately:

[0154]This satisfies the above-mentioned conditional expression (1). Thus,
in this embodiment, the light fluxes passing through the collimator lens
3 can be separated. Therefore, an appropriate aspheric surface amount can
be set in the collimator lens 3.

[0155]Accordingly, the focal difference on the surface to be scanned in
the main scanning direction between the off axis light emitting portion
1a at a vicinity of the optical axis and the off axis light emitting
portion 1b farthest in the main scanning direction can be suppressed.

[0161]Note that, in this embodiment, when a focus shift amount on the
surface to be scanned in the main scanning direction occurs by 1 mm, a
jitter amount in the main scanning direction corresponds to a writing
misregistration amount ΔY. In this case, the jitter amount in the
main scanning direction (writing misregistration amount ΔY) is
expressed as follows.

ΔY=M×W/(f1×ffθ) (5)

[0162]Therefore, the writing misregistration amount ΔY of this
embodiment has a value as follows.

ΔY=22.5×0.35/(24.9×200)=1.58 μm.

[0163]Further, in this embodiment, image writing resolution in the main
scanning direction is 1,200 dpi. Therefore, the writing misregistration
amount ΔY (1.58 μm) is equal to or smaller than 1/4 of 1 pixel
(21.2 μm), and does not affect the image.

[0164]In this embodiment, the first aperture stop (sub scanning aperture
stop) 2 is disposed between the light source unit 1 and the collimator
lens 3, and is at the position that is 4.0 mm away from the incident
surface r1 of the collimator lens 3 (surface on light source unit side).
This is because of the purpose for locating the exit pupil position of
the imaging optical system 6 in the sub scanning direction away from the
surface to be scanned so that a pitch distance in the sub scanning
direction does not change even if the surface to be scanned is shifted in
the optical axis direction.

[0165]Note that, in this embodiment, the exit pupil position in the sub
scanning direction is located on the second imaging lens 6b, and the
light fluxes emitted from the plurality of light emitting portions cross
in the sub scanning section on the second imaging lens 6b. Therefore,
optical performances of the individual light beams in the sub scanning
direction can easily meet with each other.

[0166]In this embodiment, the focal difference ΔM in the main
scanning direction on the surface to be scanned is expressed as follows.

ΔM=0.00003×(200/24.9)2=0.002 mm.

[0167]Usually, there is no problem if the focal difference ΔM in the
main scanning direction is 2 mm or smaller. However, considering
manufacturing errors and assembling errors of optical elements, the focal
difference ΔM needs to be preferably 1 mm or smaller, more
preferably 0.5 mm or smaller.

Second Embodiment

[0168]FIG. 7 illustrates a main scanning section of an incident optical
system according to a second embodiment of the present invention. In FIG.
7, the element that is the same as the element illustrated in FIG. 1 is
denoted by the same reference numeral.

[0169]The second embodiment is different from the first embodiment in that
the collimator lens 13 is a spherical lens that is rotationally
symmetrical to the optical axis, and a shape of the exit surface of the
cylindrical lens 14 in the main scanning section is noncircular. Other
structures and optical actions are the same as those of the first
embodiment.

[0170]In other words, in FIG. 7, a collimator lens 13 serves as a first
optical element, and is formed of a glass spherical lens that can be
manufactured by grinding and is a sphere including an incident surface
and an exit surface rotationally symmetrical to each other.

[0171]The plastic cylindrical lens 14 serving as the second optical
element 14 works so that the light flux that has passed through the
collimator lens 3 forms, in the sub scanning section, a linear image
elongated in the main scanning direction on the deflection surface 10a of
the light deflecting device 10.

[0172]The shape of the exit surface of the cylindrical lens 14 in the main
scanning section is a noncircular shape in which a convex (positive)
power decreases from the optical axis toward the peripheral portion. The
incident surface of the cylindrical lens 14 is plane.

[0173]In this embodiment, the generatrix of the exit surface of the
cylindrical lens 14 has an r component of zero (plane) and the aspheric
surface (noncircular) coefficient of fourth or higher order.

[0174]Table 3 illustrates values of the incident optical system in this
embodiment.

[0175]Here, the distance La in the conditional expressions (1) and (2)
shown in the first embodiment described above corresponds to an optical
path length from the noncircular exit surface of the cylindrical lens 14
in the main scanning section to the second aperture stop 5 in the second
embodiment.

[0176]In addition, the light flux width D of the light flux emitted from
the light emitting portion that is farthest from the optical axis of the
collimator lens 3 in the main scanning direction in the conditional
expression (1) corresponds to a light flux width of the light flux in the
main scanning direction on the exit surface of the cylindrical lens 14.

[0186]FIG. 8 illustrates a field curvature in the main scanning direction
of the incident optical system LA according to the second embodiment of
the present invention. In FIG. 8, numeral 800 denotes a paraxial image
surface position of the light emitting portion 1a at a vicinity of the
axis, and numeral 801 denotes a paraxial image surface position of the
farthest off axis light emitting portion 1b. In addition, the light
source unit 1 includes the 8-beam laser arranged in the one-dimensional
manner similarly to the first embodiment.

[0187]As understood from FIG. 8, a focal difference Δmcol is 0.3
μm, and a focal difference ΔM in the main scanning direction on
the surface to be scanned 7 in this embodiment can be expressed by the
following equation.

ΔM=|Δmcol|×(ffθ/fcol)2

[0188]where:

|Δmcol| denotes a field curvature difference between the light
emitting portion 1a at a vicinity of the optical axis and the light
emitting portion 1b farthest in the main scanning direction of the
incident optical system LA;ffθ denotes a focal length of the
imaging optical system 6 in the main scanning section; andfcol
denotes a focal length of the collimator lens 13.

[0189]Therefore, in this embodiment, the following equation holds so that
the focal difference ΔM in the main scanning direction on the
surface to be scanned 7 is controlled to be 0.5 mm or smaller:

ΔM=0.0003×(200/24.9)2=0.02 mm.

[0190]Note that in this embodiment, the cylindrical lens 14 has the r
component of zero, but the r component may be other value than zero so as
to be advantageous for molding. In addition, the cylindrical lens 14 may
include a diffraction element on the lens surface thereof.

[0191]Note that in this embodiment, the exit surface of the cylindrical
lens 14 is noncircular in the main scanning section, but this structure
should not be interpreted as a limitation. The incident surface or both
the surfaces may be noncircular in the main scanning section.

Third Embodiment

[0192]FIG. 10 illustrates a main scanning section of an incident optical
system according to a third embodiment of the present invention. In FIG.
10, the element that is the same as the element illustrated in FIG. 1 is
denoted by the same reference symbol.

[0193]Table 4 illustrates values of the incident optical system in this
embodiment.

[0194]The third embodiment is different from the first embodiment
described above in that a collimator lens 73 is formed of a plastic mold
lens and that the distance between the collimator lens 73 and the
cylindrical lens 4 is decreased compared with the first embodiment. In
addition, as illustrated in FIG. 12, a surface light emission laser
(VCSEL) including 64 light emitting portions arranged in a
two-dimensional manner is used as the light source unit 1. Other
structures and optical actions are the same as those of the first
embodiment, and similar effects can be obtained.

[0199]This satisfies the above-mentioned conditional expression (1)
similarly to the first embodiment.

[0200]Further, the setting is made as follows:

[0201]La=80.0 mm; f1=24.9 mm, M=22.5 mm; and L=106.26 mm.

[0202]Therefore, the following equations hold:

[0203]|La|=111.48 mm; 2f1=49.8 mm; and

M/L=22.5/106.26=0.14.

[0204]This satisfies the above-mentioned conditional expressions (2) and
(3) similarly to the first embodiment.

[0205]Therefore, compared with the first embodiment described above, the
third embodiment achieves both the short optical path of the incident
optical system and the suppressed focal difference in the main scanning
direction on the surface to be scanned 7.

[0206]FIG. 11 illustrates the field curvature in the main scanning
direction of the incident optical system LA according to the third
embodiment of the present invention. In FIG. 11, numeral 1100 denotes a
paraxial image surface position of the light emitting portion 1a at a
vicinity of the axis, and numeral 1101 denotes a paraxial image surface
position of the farthest off axis light emitting portion 1b. ΔM is
suppressed to be equal to or smaller than 0.5 mm as follows:

[0207]Δmcol=0.9 μm;

[0208]ΔM=0.0009×(200/24.9)2=0.06 mm.

[0209]Note that the collimator lens 73 in the third embodiment is made of
plastic, and hence the focal difference due to environmental change such
as temperature change becomes larger than that in the first embodiment.
However, focusing on the surface to be scanned can be compensated by
using a diffraction element or by being provided with a focal adjustment
mechanism.

[0210]In the first to third embodiments, both of the number of collimator
lenses and the number of cylindrical lenses are one, but may be plural.

[0211]In addition, the lens surface having the noncircular shape in which
the positive power continuously decreases from the axis toward the off
axis in the main scanning section may be disposed to each of the
collimator lens and the cylindrical lens.

[0212][Color Image Forming Apparatus]

[0213]FIG. 13 is a main portion sectional view of a color image forming
apparatus according to an embodiment of the present invention. The color
image forming apparatus of this embodiment is of tandem type, which
includes four optical scanning apparatuses (optical imaging systems)
arranged side by side to record concurrently image information on
surfaces of photosensitive drums, which serve as image bearing members.
FIG. 13 illustrates a color image forming apparatus 60, optical scanning
apparatuses 61, 62, 63, and 64 structured as illustrated in any one of
the first to third embodiments, photosensitive drums 21, 22, 23, and 24
serving as image bearing members, developing devices 31, 32, 33, and 34,
and a conveyor belt 51. It should be noted that, in FIG. 13, there are
provided a transferring device (not shown) for transferring a toner image
developed by the developing device onto a transfer material, and a fixing
device (not shown) for fixing the transferred toner image on the transfer
material.

[0214]In FIG. 13, respective color signals of red (R), green (G), and blue
(B) are input from an external device 52 such as a personal computer to
the color image forming apparatus 60. The color signals are converted
into pieces of image data (dot data) of cyan (C), magenta (M), yellow
(Y), and black (B) by a printer controller 53 in the color image forming
apparatus. The respective pieces of image data are input to the optical
scanning apparatuses 61, 62, 63, and 64. Light beams 41, 42, 43, and 44,
which are modulated according to the respective pieces of image data, are
emitted from the optical scanning apparatuses. The photosensitive
surfaces of the photosensitive drums 21, 22, 23, and 24 are scanned with
the light beams in a main scanning direction.

[0215]In the color image forming apparatus of this embodiment, the four
optical scanning apparatuses 61, 62, 63, and 64 are arranged side by
side, each corresponding to the respective colors of cyan (C), magenta
(M), yellow (Y), and black (B). The optical scanning apparatuses
concurrently record the image signals (image information) on the surfaces
of the photosensitive drums 21, 22, 23, and 24, and print a color image
at high speed.

[0216]As described above, the color image forming apparatus of this
embodiment uses the light beams which are respectively based on image
data and emitted from the four optical scanning apparatuses 61, 62, 63,
and 64 to form latent images of four colors on the surfaces of the
photosensitive drums 21, 22, 23, and 24 respectively associated with the
four colors. The latent images are then transferred to a recording
material one on another through multilayer transfer to form one full
color image.

[0217]The external device 52 may be a color image reading device including
a CCD sensor. In this case, the color image reading device and the color
image forming apparatus 60 constitute a color digital copying machine.

[0218]While the present invention has been described with reference to
exemplary embodiments, it is to be understood that the invention is not
limited to the disclosed exemplary embodiments. The scope of the
following claims is to be accorded the broadest interpretation so as to
encompass all such modifications and equivalent structures and functions.

[0219]This application claims the benefit of Japanese Patent Application
No. 2008-287639, filed Nov. 10, 2008, which is hereby incorporated by
reference herein in its entirety.